In order to cool the temperature of electronic components, aluminum heat sinks are often used. Aluminum and/or aluminum alloys are often utilized to form heat sinks due to their high thermal performance characteristics, ability to be extruded and availability in the marketplace. A cost efficient way of manufacturing an aluminum heat sink is extruding the metal in a fin and base configuration as shown in FIGS. 1a-1d. 
In the embodiment shown in FIG. 1 and disclosed in U.S. Pat. No. 6,637,109 B2, the heat sink has base portions 15 and 17 that act as a mounting surface for the heat generating components, and the fins therebetween form a fin portion which adds a surface area, which is critical for maximizing heat dissipation from attached heat-generating components. Heat that is generated by the components is initially transferred through conduction to the base portions 15 and 17 (top and bottom portions) of the heat sink and subsequently into the fins where the heat is dissipated to the surroundings via radiation, forced convection, and/or natural convection. Because there is a direct correlation between the amount of surface area and total heat transfer, it is advantageous to maximize fin height and density, which minimizes the gap/space therein between fins, particularly when dealing with forced convection applications.
Aspect ratio is a relationship for characterizing heat sinks, where the ratio is mathematically defined as fin height divided by the gap between fins. Current extrusion technology limits the ratios achievable because as the aspect ratio increases, the extrusion die becomes weaker when extruding such heat sinks as a one-piece profile. To decrease the probability of die failure, the extrusion process speed is usually decreased, reducing overall productivity of the extrusion press. Heat sinks with higher ratio fins manufactured as one-piece by the aluminum alloy extrusion method also incur larger scrap rates.
One method of addressing issues related to manufacturing of high-aspect ratio heat sinks from single extruded profiles is joining them together by Friction Stir Welding (FSW) as disclosed in U.S. Pat. No. 6,637,109 B2. The method includes extruding or bending of an extruded profile which is cut in a plurality of pieces of the appropriate length. The single profile cross-section includes a first end portion, a second end portion, and a connecting web portion, where the first and second base portions are thicker than the web portion. The pieces of extruded profiles are then aligned and joined together by friction stir welding along their contacting surfaces. In cases where heat generating components are placed on both the top surface of the base portion 15 and the bottom surface of the base portion 17, this can be a cost effective solution that overcomes the problem of extruding a single piece as noted above.
A drawback of this method is that it cannot be utilized to form a heat sink having a base plate for mounting the element to be cooled on only one side. This problem occurs when such a “one-sided” heat sink with very dense fins is needed. It is not possible to extrude a one-sided heat sink with dense fins as a single-piece unit due to previously mentioned limitations surrounding the aluminum extrusion process.
Thermal performance of heat sinks generally increases as the ratio of fin height to fin gap increases. As a result, bonded heat sinks are used when high heat load dissipation is required. Bonding techniques for making bonded heat sinks include brazing, epoxy bonding, mechanical (press-fit or snap), and friction stir welding. However, while high fin density heat sinks can be made by brazing and epoxy bonding, a filler material has to be used to facilitate the bond, which creates some degree of thermal resistance at the joint, which has a negative effect on overall thermal performance. Additionally, these methods are labor intensive, which causes their manufacturing costs to be quite high. Furthermore, it is known that epoxy joints can weaken over time, reducing the mechanical strength of the bond and inhibiting heat transfer.
Mechanical joining includes making heat sinks from extrusions, profiles, called lamellar, or other segments having a number of fins and interconnected by press and/or snap fitting. However, while this method is cost-efficient, it suffers from a few disadvantages. The disadvantages of mechanical joining include a mediocre bonding strength, which is not suitable for all applications. This can cause reliability issues during exploitation of such a heat sink. Additionally, air gaps can exist around mechanical joints, which can cause high spreading thermal resistance. Furthermore, machining and drilling processes, which are required for heat pipe integration and/or heat generating component attachment, may loosen the adjacent mechanical joints of two neighboring extrusions or profiles.
Friction stir welding has also been utilized as a bonding method to join aligned extrusions or lamellar segments. Friction stir welding for joining parts made of aluminum alloy typically utilizes a non-consumable rotating tool including a shoulder and a pin, which often includes specially configured surfaces for increasing friction when the tool is in contact with the metal. The rotating friction stir welding tool usually also moves linearly along the adjacently aligned edges of two workpieces to be joined together. The friction generated by this rotation heats and plasticizes the material at the weld zone. The plasticized material of two joining adjacent parts is fused together and thus creates a weld seam along their edges. As the tool rotates, a downward force is applied on the workpiece to sufficiently fuse the two pieces together. Deformation might occur when the rotating tool traverses abutting edges of lamellar segments or profiles since there is no support or joint underneath for the one-sided heat sink embodiment to counteract the previously mentioned down force. Therefore, the workpieces usually require a support from the side opposite the welding where the force is applied, a rear or down side, as, for example, is shown in FIG. 1a. Alternatively, the workpieces have a shape allowing the other, down or opposite part of the work piece to serve as a support to compensate for the rather substantial vertical forces applied during friction stir welding. It is known in the art to use friction stir welding for joining of two metal or plastic work pieces.
FSW is the most efficient bonding method providing high tensile strength of the joined parts while maintaining high thermal conductivity at the joint. However, when forming heat sink segments from lamellar or extruded profiles, the segments are typically arranged only by clamping. When the segment is formed by, for example, a press-fit connection, where a neighboring extrusions or profiles have interacting extensions on one side and grooves receiving those extensions on the other side, significant air gaps might exist between lamellar segments in those press-fit connections around extensions spaced within the grooves in non-weld areas, especially for a large scale heat sink. Such gaps can result in high spreading thermal resistance. Additionally, if these air gaps occur in the weld zone, they can create a defective weld that will contain voids. In addition, I-, U-, and S-shaped cross-sections, such as those shown in FIGS. 1b, 1c and 1d, need to include two end portions, which limits the variations of shapes of the final product and increases the weight of the heat sink.